Secondary Flow in a Bend

Secondary flows are due to streamwise vorticity, which are caused by the interaction between pressure gradients normal to the streamline and boundary layers.

In a bend is according to the radial-pressure-equation (pressure gradient due to the curvature) the pressure at the outside of the bend greater than the pressure at the inside. These pressure gradients interact with boundary layers and result in a secondary flow and produce a double vortex - with boundary layer wall jets - which transfers flows at the boundary layers of the upper und lower side walls from pressure side to suction side.